Option D is the right answer as ‘deb’ is the correct sequence of prepositions to be used in the given set of sentences.
As we can see:
1. After the burglary, the office bought a sophisticated security system as an insurance against any such losses. We use against to refer to negative, hostile or opposing actions to an event.
2. She put up a brave front at the end of the meeting. Put up refers to exhibiting determination or strength in a difficult situation.
3. Sunil decided to avail himself of the opportunity to go to Australia for training. To avail oneself of something is to take advantage of it.
So, the correct answer is D.